1. Field of the Invention
The present invention relates to an image detection device for obtaining images by converting light signals into electrical signals and more particularly to an image detection device of a medical X-ray diagnostic system.
2. Discussion of Background
An image detection device using photoelectric converting elements as image detection elements is being widely used in video cameras, digital cameras and the like. It is also used in a medical X-ray diagnostic system instead of the conventional screen films.
The medical field is now trending toward making a data base of medical data of patients in order to be able to quickly and accurately give treatment. It is then required to make a data base of picture data obtained by X-ray photography and to digitize photographed X-ray pictures.
The conventional screen film has been used in the medical X-ray diagnostic system to take diagnostic pictures. However, it is necessary to develop the photographed film and then to scan the pictures by a scanner or the like to digitize the pictures, requiring considerable work and time. This method has had another problem that the image quality of the picture is lowered when scanned by the scanner.
A method of using a charge coupled device (CCD) camera whose size is as small as 1 inch square and of photographing directly to obtain digital images has been realized recently.
However, because it is necessary to photograph an area of about 30 cm.times.30 cm in photographing a human lung for example, it requires an optical apparatus for condensing light, causing a problem that the system is enlarged.
As a method for solving these problems, an image detection device using thin film transistors (TFT) using amorphous silicon (a-Si) as a semiconductor film has been proposed (e.g., U.S. Pat. No. 4,689,487).
There are two types of a-Si TFT image detection devices for converting X-rays into electric charges. One is called an indirect conversion image detection device which converts X-rays once into visible light by fluorescent substance or the like and then converts the visible light into electrical charges by means of a photoelectric converting film. The other is called a direct conversion image detection device which converts X-rays directly into electric charges by means of a photoelectric converting film.
While the indirect conversion image detection device allows to obtain pictures by converting X-rays into visible light by the fluorescent substance, it has a drawback that it is difficult to obtain enough spatial resolution because light scatters within the fluorescent substance.
Meanwhile, the direct conversion image detection device has merit in that it allows high spatial resolution image quality to be obtained because it requires no fluorescent substance which causes the deterioration of the spatial resolution. It is essential to be able to obtain high spatial resolution pictures for the purpose of medical use, so that the direct conversion image detection device is now drawing attention.
FIG. 9 is a schematic block diagram showing an overall system utilizing the image detection device. High voltage is supplied from a high voltage generating section 62 to an X-ray source 51. X-rays irradiated from the X-ray source 51 penetrate through a specimen 52 and enter photoelectric converting elements of an a-Si TFT image detection device 53. The a-Si TFT image detection device 53 converts the X-rays penetrated through the specimen 52 into an analog electrical signal corresponding to a dosage at the incident position of the photoelectric converting elements. The converted analog signal is then digitized by an A/D converter 57 to be stored in an image memory 58 in a time series manner.
The image memory 58 is capable of storing image data of one or several pictures sequentially at predetermined addresses under the control of control signals from a control section 63. An arithmetic processing section 59 implements arithmetic processing on the image data by taking it out of the image memory 58 so that it can be displayed appropriately and stores the result thereof again in the image memory 58. The processed image data in the image memory 58 is then converted into an analog signal by a D/A converter 60. This analog signal is output to an external processing circuit such as an image monitor 61 via an interface. Accordingly, an X-ray penetration image of the specimen 52 is displayed, for example, on the image monitor 61. The control section 63 also controls the high voltage generating section 62.
FIG. 10 is a schematic diagram showing the structure of the image detection section of the direct conversion image detection device using the a-Si TFTs. As shown in FIG. 10, pixels e, each of which is a unit element composing an image detection area, are arrayed in a matrix of 2000 (H).times.2000 (V) for example (hereinafter referred to as a thin film transistor array).
Bias voltage is applied to a photoelectric converting film 140 from a power source 148. A drain of an a-Si TFT 144 is connected to a signal line 113. A gate of the a-Si TFT 144 is connected to a scan line 118. ON/OFF of the a-Si TFT 144 is controlled by potential of a scan signal applied from a scan line driving circuit 152 to a gate electrode via the scan line 118. A terminal end of the signal line 113 is connected to an amplifier 154 such as a sense amplifier for detecting signals.
FIGS. 11(a) and 11(b) are schematic diagrams showing a basic structure of the direct conversion image detection device, wherein FIG. 11(a) is a schematic section view of the image detection device and FIG. 11(b) is a diagram schematically showing an equivalent circuit of the unit pixel of the image detection device. Each pixel e comprises the thin film transistor 144 composed of the semiconductor film of a-Si, the photoelectric converting film 140 and a signal storage capacitor element Cs.
When light (X-rays, soft X-rays or the like in this case) enters the photoelectric converting film 140, a current flows through the photoelectric converting film 140 and electric charge is accumulated in a pixel capacitor Cs. When the scan line driving circuit 152 drives one scan line to turn ON all TFTs connected to the scan line, the accumulated charge is transferred to the amplifier 154 side via the signal line 113. An output amplitude of the amplifier 154 also changes corresponding to a difference in amounts of charge caused by a difference in quantities of light entering the pixels. The method shown in FIG. 10 allows a digital image to be obtained directly by converting the output signal of the amplifier 154 from analog to digital form.
The pixel shown in FIGS. 11(a) and 11(b) has a similar structure with an active matrix liquid crystal display using thin film transistors as switching elements (TFT-LCD), which is used for a notebook (laptop) personal computer and the like. Accordingly, a thin and large screen image detection device may be readily fabricated.
However, it is necessary to form the photoelectric converting film as thick as about several hundreds .mu.m to several mm in the direct conversion image detection device in order to improve the efficiency of the conversion from X-rays to electric charges. Then, in order to apply an appropriate electric field to the photoelectric converting film, a high voltage of about several kV must be applied on the both sides of the photoelectric converting film.
As shown in FIGS. 11(a) and 11(b), the image detection area of the direct conversion image detection device includes the a-Si TFT 144, a storage capacity electrode 202, a pixel electrode 203, an insulating layer 204, the photoelectric converting film 140 and a voltage supplying wire 205 disposed on an insulated substrate 201. Also provided are the signal line 113 and the scan line 118 as shown in FIG. 10.
The signal storage capacitor element Cs is formed by pinching the insulating layer 204 between the storage capacitor electrode 202 and the pixel electrode 203.
A high voltage of about several kV is applied to the voltage supplying wire 205.
The resistance of the photoelectric converting film 140 decreases in proportion to the dosage of X-rays entering therein. Accordingly, when a predetermined dosage of X-ray enters the photoelectric converting film 140, a current flows between the voltage supplying wire 205 and the pixel electrode 203 and electric charge corresponding to the dosage of the X-rays is stored in the signal storage capacitor element Cs.
Because the electric charge stored in each pixel is proportional to the dosage of X-rays entering each pixel, the X-ray penetration image corresponding to the specimen may be detected as electric charges by arraying such image detection pixels two-dimensionally.
After accumulating the charges for a certain period of time, the scan line driving circuit 152 turns ON the a-Si TFT 144 to read the charge accumulated in the signal storage capacitor element Cs through the amplifier 154 to output to the circuit.
In the direct conversion image detection device having such structure, however, a large current flows through the photoelectric converting film 140 when intense X-rays enter the photoelectric converting film 140 and the potential of the pixel electrode 203 rises up to about several kV which is equal to that of the voltage supplying wire 205 in the worst case. When a large potential difference of several kV is generated between the pixel electrode 203 and the storage capacity electrode 202, there is a great possibility that the insulating layer 204 experiences dielectric breakdown, thus destroying the TFT array.
Accordingly, it has become necessary to suppress the potential of the pixel electrode 203 below a certain voltage and include a protection circuit within the pixel, as has been proposed in Japanese Patent Application Nos. Hei. 8-161977 and Hei. 8-32699 for example.
FIG. 12 is a diagram showing a peripheral edge portion of this image detection device. A high voltage of about several kV is applied also to a part other than the image detection area and there is a possibility that dielectric breakdown occurs in that part, thus destroying the thin film transistor array of the image detection area.
A high-voltage power cable 307 is connected to the voltage supplying wire 205 to apply the high voltage in the peripheral edge portion of the image detection area.
Normally, an electrodes--the scan line 118, the signal line 113 for reading signals, the storage capacity electrode 202 or the like--for driving the pixel section is led to the outer peripheral part of the substrate in order to be connected to the scan line driving circuit 152 and the amplifier 154, respectively. Accordingly, electrostatic capacitors C140, C206 and C204 are formed between the electrode 305 for driving the pixel section and the voltage supplying wire 205. Therefore, when X-rays enter the peripheral edge portion of the photoelectric converting film 140, a current flows between the voltage supplying wire 205 and the electrodes similarly to the image detection area. However, dielectric breakdown may occur and the TFT array of the image detection area may be destroyed when a high voltage of several kV is applied to the insulating layer 204 and the insulating film 206.
As described above, the conventional image detection device has had the problem that the insulating layer 204 and the insulating film 206 experience dielectric breakdown, thus destroying the image detection device, even in the peripheral portion of the image detection area similarly to the image detection area when high voltage is applied between the wires such as the scan line 118, the signal line 113 and the storage capacitor electrode 202 and the voltage supplying wire 205.